Organic light emitting diodes (OLED) comprise at least one layer of an organic material which emits light in the visible spectral range upon application of an external voltage. The organic layer, also known as active layer, is sandwiched between two electrode layers in order to apply the necessary voltage for light emission. At least one of these electrode layers is of an electrically conducting material which is at least partly transparent for wavelengths of the light emitted by the active layer. It is also known to arrange one or several further layers between the electrode layers and the active layer. These additional layers may for example serve as electron or hole transport or injection layers. The construction of such a OLED light source is similar to a parallel-plate capacitor with the active material between the two electrode plates.
There is a need to provide large area LED light sources with lateral extensions of the active layer and electrode layers ranging from several centimeters to more than one meter. The active layer in an OLED light source is typically 100 nm or even less in thickness. Due to this small thickness and to the roughness and irregularities of the underlying base substrate, on which the layers are arranged, local or small area electrical shorts may occur between the two electrode layers. Further reasons of such electrical shorts are the application of a too high voltage between the electrodes or the aging of the layer structure. The leakage current caused by an electrical short inhibits the injection of electrons into the active layer so that this layer in a large area up to the whole area of the light source stops emitting light. The probability and number of shorts is proportional to the geometrical area of the light source, i.e. the active layer, which is in particular problematic for large area OLED or LED light sources.
U.S. Pat. No. 6,870,196 B2 discloses a OLED light source comprising a plurality of groups of OLED segments, the OLED segments in each group being electrically connected in parallel and the groups being electrical connected in series. A fuse is connected in series with each OLED segment for opening the electrical connection to an OLED segment in response to an electrical short. Due to the series/parallel connection of the OLED segments of this light source, the occurrence of an electrical short only influences one segment of the light source, the corresponding OLED segment, whereas the remaining segments remain unaffected and continue to emit light.